Printing plate



Patented VMar. 26, 1946 UNITED STATES PATENT OFFICE PRINTING PLATE John J. Murray, Arlington, Mass.

1 Claim.

This invention relates to an improved printing plate.

In those printing processes in which the printing is accomplished from a plane surface, and the non-printing surfaces are removed by etching, these processes being generally known as relief processes, it is quite frequently necessary to increase the contrast between the dark and light portions of the picture. In the present practice, this contrast is obtained by coating by hand the darker portions of the otherwise finished plate with a resist and then further etching the exposed parts. In complex pictures, this process is quite impracticable, and, in simple pictures, it is expensive. The present method affords automatic control for such processes and the plates produced by following it are not only better than those previously produced, but are readily distinguishable therefrom because of the pyramidal shape of the dots. In following the present process, the relief printing procedure is followed up to and including the etching of the plate in the normal manner to produce the ordinary unimproved printing surface. At this stage, the plate consists of a metal such as copper or zinc which is still covered in the unetched portions with a resist which is generally termed an enamel and is the remains of the originally photosensitive film. The etched portions are so arranged as to leave the unetched parts in a series of dots or lines.

In accordance with this process, the half-tone plate is placed in an electrolytic bath, for example, one capable of depositing chromium, copper or other metal upon the surface of the exposed areas of the plate, and current is then passed to deposit metal on the plate. The amount of metal deposited should be varied to meet the individual requirements of the picture; but ordinarily a time of 3 seconds to 2 minutes is employed, depending upon the strength. of the solution, the current density, and the thickness of coating desired.

At the conclusion of the plating operation it will be found that the plating metal has deposited to a greater extent upon the darker or shadowy portions of the plate, and to the least extent upon the most exposed areas. This is apparently due to the fact that the current density is heaviest in proportion to area where the amount of exposed metal is least. That is, the presence of large relative amounts of enamel in the shadowy areas acts to direct the current in that vicinity toward the relatively small amount of exposed (Cl. lOl-401.2)

are the first to be acted upon by the acid. The

acid begins its action at the edges of the enamel and works downwardly and also toward the center of the dot, leaving a dot which is pyramidshaped.

As a result of the process the contrast between the shadows and the exposed portions of the film is greatly enhanced and this enhancement is carvred out in an entirely automatic fashion and very inexpensively.

After the final etching has been completed, the

' metal deposit may be removed or left upon the plate, the printing being unaffected by the metal if it is desired to leave it on.

The invention is illustrated diagrammatically in the drawing in which Figure 1 shows a crosssection of a portion of a plate I I) comprising a series of dots I I covered at the top with remnants of enamel I2; in Figure 2 the same plate is shown after the deposit of the metal I3; and in Figure 3 the plate is shown after the final etching, leaving the pyramidal shaped dots I4.

This application is a division of my co-pending application Serial No. 337,509, led May 27, 1940, now Patent No. 2,367,236.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom,

What I claim as new, and desire to secure by 40 Letters Patent, is:

A plate for relief printing comprising a metal base, a reproduced relief surface thereon consisting of relief portions and adjacent depressions, the relief portions being capped with a resist and the ratio of depressed area to relief area being variable over the face of the plate, and an electrodeposited thin layer of etchable metal on the depression walls, the thickness of the electrodeposited layer varying inversely with the ratio of relief area to depressed area in any given unit oi area over the face of the plate.

JOHN J. MURRAY. 

